Fair Value Measurements - Financial Instruments Not Reported at Fair Value (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Assets (liabilities)
Mortgage loans	$ 14,919.7	$ 14,150.5
Policy loans	804.4	808.3
Short-term debt	(36.1)	(39.5)
Long-term debt	(3,245.5)	(3,178.4)
Carrying amount
Assets (liabilities)
Mortgage loans	14,919.7	14,150.5
Policy loans	804.4	808.3
Other investments	247.4	236.8
Cash and cash equivalents not required to be reported at fair value	1,123.7	677.5
Investment contracts	(32,863.9)	(31,003.0)
Short-term debt	(36.1)	(39.5)
Long-term debt	(3,245.5)	(3,178.4)
Separate account liabilities	(147,429.4)	(145,552.5)
Bank deposits	(515.4)	(2,336.4)
Cash collateral payable	(61.6)	(125.8)
Assets (liabilities) measured at fair value
Assets (liabilities)
Mortgage loans	14,855.1	14,443.2
Policy loans	939.2	998.4
Other investments	239.7	234.0
Cash and cash equivalents not required to be reported at fair value	1,123.7	677.5
Investment contracts	(31,651.5)	(30,468.8)
Short-term debt	(36.1)	(39.5)
Long-term debt	(3,275.5)	(3,442.5)
Separate account liabilities	(146,205.9)	(144,300.5)
Bank deposits	(502.5)	(2,328.9)
Cash collateral payable	(61.6)	(125.8)
Assets (liabilities) measured at fair value \| Fair value hierarchy Level 1
Assets (liabilities)
Cash and cash equivalents not required to be reported at fair value	1,070.9	659.9
Bank deposits		(1,780.3)
Cash collateral payable	(61.6)	(125.8)
Assets (liabilities) measured at fair value \| Fair value hierarchy Level 2
Assets (liabilities)
Other investments	156.5	159.4
Cash and cash equivalents not required to be reported at fair value	52.8	17.6
Investment contracts	(4,647.0)	(4,736.1)
Short-term debt	(36.1)	(39.5)
Long-term debt	(3,161.5)	(3,393.5)
Bank deposits	(502.5)	(548.6)
Assets (liabilities) measured at fair value \| Fair value hierarchy Level 3
Assets (liabilities)
Mortgage loans	14,855.1	14,443.2
Policy loans	939.2	998.4
Other investments	83.2	74.6
Investment contracts	(27,004.5)	(25,732.7)
Long-term debt	(114.0)	(49.0)
Separate account liabilities	$ (146,205.9)	$ (144,300.5)

Includes currency on hand and other cash equivalents not required to be reported at fair value.

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This item represents the obligation to return cash received as collateral from counterparties.

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This item represents investment contracts such as guaranteed investment contracts and funding arrangements that do not have significant insurance risk. This item excludes any embedded derivatives within investment host contracts that are required to be reported at fair value.

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This item represents invested assets not specifically identified in the statement of financial position, excluding derivative assets. This includes invested assets such as seed money and commercial mortgage loans of consolidated variable interest entities reported at fair value.

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The aggregate of all deposit liabilities held by the entity, including foreign and domestic, interest and noninterest bearing; may include demand deposits, saving deposits, Negotiable Order of Withdrawal (NOW) and time deposits among others.

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Amount before allowance of loans made to policy and contract holders using their life insurance contracts' cash value or separate accounts as collateral. Includes deferred interest and fees, undisbursed portion of loan balance, unamortized costs and premiums over and discounts from face amounts. Excludes loans and leases covered under loss sharing agreements.

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Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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The balance represents the amount of loans that are secured by real estate mortgages, offset by the reserve to cover probable credit losses on the loan portfolio.

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Reflects the total carrying amount as of the balance sheet date of debt having initial terms less than one year or the normal operating cycle, if longer.

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